1. Field of the Invention
The invention relates to an arrangement for demodulating an FM modulated signal, comprising an input terminal for receiving the FM modulated signal and an output terminal for supplying a demodulated signal, having a signal path between the input and output terminals, constituted by:
a demodulator circuit for demodulating the FM modulated signal, having an input coupled to the input terminal, and an output, PA1 a hold circuit, having a signal input coupled to the output of the demodulator circuit, a control signal input and an output which is coupled to the output terminal, this hold circuit at least comprising a first controllable switching means, having an input and an output coupled to the input or output, respectively, of the hold circuit, and a control input coupled to the control signal input of the hold circuit, and a capacitor inserted between the output of the hold circuit and a first point of constant potential, the hold circuit being devised for at least substantially holding the signal applied to its input in response to a control signal applied to the control signal input and the first switching means being devised for interrupting for a first time interval T.sub.1 the internal connection between the switch input and output in response to this control signal. PA1 two or more read heads disposed on a rotatable head drum, PA1 a switching means having two or more inputs, an output and a control input, which switching means is further arranged for coupling one of the inputs to the output in response to a control signal to be applied to the control input, an output of each of the two or more read heads being coupled to an associated input of the two or more inputs of the switching means, and the output of the switching means being coupled to the input of the demodulator circuit.
Such an arrangement may be included in, for example, a videotape recorder for demodulating an audio signal recorded FM modulated in the tracks on a record carrier once the record carrier has been read out, which arrangement further includes for this purpose:
The arrangement is intended for obviating instantaneous disturbances in the input signal of the demodulator circuit. Instantaneous disturbances are the disturbances affecting the regularity in the input signal. In this context they may be, for example, drop-outs or drop-ins in the input signal of the demodulator circuit, or phase jumps in this input signal which occur as a result of the switching from record carrier reading with one read head to record carrier reading with another read head (so-called head switch). These instantaneous disturbances cause the demodulator circuit to be deranged so that (audible) disturbances occur in the signal demodulated by the demodulator circuit.
2. Description of the Related Art
The low-pass filter inserted directly after the demodulator circuit in the prior art arrangement has for its object to eliminate by filtering the disturbing signal components such as intermodulation products, carrier residuals and noise beyond the audio band, which are formed in the demodulation circuit. A typical value for the cut-off frequency of such filter is, for example, 100 to 150 kHz. The drawback of this low-pass filter is the fact that the disturbances at the output of the demodulator circuit are extended to larger lengths with time. For example, disturbances that result from a head switch and which may have a length of about 4.5 .mu.s, are extended to a length of about 8 to 10 .mu.s by a low-pass filter having a cut-off frequency of the order of 100 kHz. This implies that the first switching means is (to be) open during the time interval T.sub.1, which is approximately equal to these 8 to 10 .mu.s. The purely periodical nature of the energizing of the hold circuit (with attendant opening of the switching means) with the frequency of the head switch, in combination with the great length of the time interval T.sub.1, causes a disturbing contribution to occur in the output signal of the arrangement.